1. Field of Invention
The invention relates to an ink jet printer that performs printing by ejecting ink from a print head and, more particularly, to an ink jet printer of ink tube supply type where ink to be ejected is supplied from an ink tank to a print head through an ink supply tube.
2. Description of Related Art
A conventionally known ink jet printer of ink tube supply type is disclosed in Japanese Laid-Open Patent Publication No. 59-73953.
Such an ink jet printer is designated by the numeral 620 in FIG. 25. FIG. 25 is a perspective view showing the ink jet printer 620 in schematic form. The ink jet printer 620 includes a print head unit 621, a platen roller 622, a carriage 623, an ink tank 624, an ink tube 625, a signal input line 626, and guide rods 627.
The print head unit 621 is equipped with a print head having ink nozzles from which ink is ejected to perform printing onto a paper sheet based on a signal sent via the signal input line 626. The print head unit 621 receives a supply of ink from the ink tank 624 via the ink tube 625. The print head unit 621 is installed on the carriage 623, which is attached to a belt. The belt is looped around a roller attached to a motor. Thus, when the motor runs, the belt is driven, which allows the carriage 623 with the print head unit 621 to move along the guide rods 27 for whatever distance the belt is driven.
The guide rods 627 are slidably inserted into the carriage 623 and support the carriage 623 for movement. Thus, the print head unit 621 installed on the carriage 623 can reciprocate in a direction parallel to the guide rods 627, that is, in both directions along the length of the platen roller 622.
In the ink jet printer of ink tube supply type, when the carriage equipped with the print head performs a printing operation while moving reciprocally, the ink in the ink tube undergoes acceleration, causing pressure waves that propagate in the ink toward the print head. The above-described printer is structured to absorb the pressure waves generated in the ink by placing an air-filled damper between the tubes.
On the other hand, to hold printing quality in the proper condition, the ink jet printer needs to maintain a constant state of ink to be ejected from the ink nozzles. To maintain the state of ink to be ejected, a meniscus (curved surface) is formed on the surface of the ink at the end of each of the ink nozzles. As the meniscus is formed by, for example, causing a negative pressure in the ink to be supplied to the nozzles within a specified range, the pressure to supply the ink is kept constant, always producing the meniscus with the same shape. Thus, the ink to be ejected is maintained under a certain condition.
However, when the pressure waves propagating toward the print head are generated in the ink due to acceleration working on the ink, the negative pressure applied to the nozzles changes and the meniscuses are destroyed. Therefore, the ink to be ejected cannot be maintained under a constant condition, affecting printing quality.
Further, air may be introduced into the ink tube or the connecting portion between the damper and the ink tube, for example, when an ink tank is changed. When air is delivered by the ink flow to the ink nozzles through the ink tubes and other parts, the ink passage is closed by an air bubble when such an air bubble becomes large. Therefore, the ink jet printer is provided with a purging operation that is a process to recover the state of the ink to be ejected from the ink nozzles.
However, when discharging the air, ink is also discharged. This causes not only a wasting of ink but also a decrease in the efficiency of the air purging operation.